The used value plus the available value
equals the total swap space on the system, which includes a portion of physical
memory and swap devices (or files).

You can use the amount of available and used swap space (in the swap
-s output) as a way to monitor swap space usage over time. If a
system's performance is good, use swap -s to determine
how much swap space is available. When the performance of a system slows down,
check the amount of available swap space to determine if it has decreased.
Then you can identify what changes to the system might have caused swap space
usage to increase.

When using this command, keep in mind that the amount of physical memory
available for swap usage changes dynamically as the kernel and user processes
lock down and release physical memory.

Note –

The swap -l command displays swap space in
512-byte blocks. The swap -s command displays swap space
in 1024-byte blocks. If you add up the blocks from swap -l and
convert them to KB, the result is less than used + available (in the swap -s output). The reason is that swap -l does not include physical memory in its calculation of swap
space.

The output from the swap -s command is summarized
in the following table.

Table 20–1 Output of the swap
-s Command

Keyword

Description

bytes allocated

The total amount of swap space in 1024-byte blocks that is currently
allocated as backing store (disk-backed swap space).

reserved

The total amount of swap space in 1024-byte blocks that is not currently
allocated, but claimed by memory for possible future use.

used

The total amount of swap space in 1024-byte blocks that is either allocated
or reserved.

available

The total amount of swap space in 1024-byte blocks that is currently
available for future reservation and allocation.